This invention relates generally to batch type polymer mixers and more particularly to a kneader mixer with intermeshing rotors.
There are two different types of internal mixers used in the industry at large. The first type is more commonly known as a “Banbury” type intensive mixer and the second type is known as a “Kneader”. The primary difference between the two types of mixers is the rotor, throat, chamber and floating weight design. Banbury® mixers also discharge the batch through a bottom door where as the kneader tilts to discharge the batch.
A closed kneader is composed of a kneading tank or chamber for holding kneaded material, a pair of rotors, which are provided at both ends with rotor drive shafts passing through side walls of the chamber, and which consist of a rotor shaft forming a rotor blade for kneading the material kneaded in the chamber, and a pressure cover. When kneading material, the kneaded material is poured into the chamber when the pressure cover is opened upward. The pressure cover is let down and the rotors are rotatably driven with a driving means such as a motor, etc., connected, to the rotor drive shafts.
Traditional kneaders have two counter rotating rotors with each mixing rotor having two thin wings affixed on it. The two wing rotors typically rotate at different speeds through connecting gears. The wings move material from one portion of the chamber to the other while also providing material movement along the rotor axis. These kneaders do not have intermeshing rotors and therefore can have differential rotor speeds.
Typically, conventional kneaders have a one piece rotor design that includes a rotor shaft with two wings welded on the shaft. The conventional kneader's blades are typically long, high and narrow. Water cooling is provided through a passage in the rotor shaft and small jackets in each wing. This cooling method is usually not sufficient for single pass mixing.
During kneading, heat is produced within the kneaded material, in the chamber and also by the rotor shaft because of internal heat generation due to shearing, dispersion, etc., during kneading. For these reasons, insufficient cooling often occurs in the inner part of the kneaded material with respect to the kneading speed, i.e., the speed of heat generation, in the case of a kneaded material of large thickness and low thermal conductivity (especially rubber, etc.), even if cooling water is circulated through the chamber wall and the rotor shaft.
While sufficient cooling can be provided for the kneaded material with little increase of internal temperature in the kneaded material by using a kneader with a small mixing volume, such a machine is inferior in productivity and therefore unrealistic as a mass production unit.
In the closed kneader, while a general kneading process is divided into a primary kneading for mixing without containing any vulcanizing agent, and a secondary kneading for performing kneading by mixing the kneaded material, which has already been subjected to a certain kneading process in the primary kneading, with a vulcanizing agent, the kneading material temperature must be kept below a certain level (variable depending on the material) for mixing in the vulcanizing agent.
Generally, convention kneaders require processing a batch in two passes. Either by stopping the mixing and allowing the batch to cool before completing the mixing, or by discharging the batch into a second kneader.
Conventional kneaders typically use pneumatic pressure to push the batch down into the rotors and mixing chamber with a floating weight (ram). This pneumatic system can be unreliable and inconsistent. The pneumatic ram moves uncontrollably with the ram position being typically controlled by the rotor dragging force as well as the size of rubber or polymer pieces being forced into the rotors.
The foregoing illustrates limitations known to exist in present kneader mixers. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.